Millions of dollars are expended annually by communities attempting to isolate and eliminate storm water inflows to their sanitary sewer systems. For many years, the engineering profession has recognized that accurate, temporary measurement of wastewater flows emanating from subareas within a total sewer system is the most cost-effective means of determining the portions of the system with the most serious inflow problems.
Sewers that carry wastewater from areas that experience severe storm water inflow problems typically operate under surcharged conditions during and immediately after rainfall events. Unfortunately, it is under these conditions that accurate flow rate data are almost impossible to obtain. For a variety of reasons, it is normally impractical to use a primary flow metering device, such as a weir or flume, that would allow depths of flow to be converted into flow rates. Weirs are subject to upstream sedimentation and fouling by debris; they must be fabricated to suit the physical configuration of each particular manhole, and are difficult to calibrate under surcharged conditions. Flumes, such as the Palmer Bowlus flume and other venturi flumes, are typically inaccurate at upstream depths of flow that exceed 75 percent of the sewer diameter and are useless under surcharged conditions. An alternative is to measure the head loss (usually fractions of an inch) between two manholes and to use culvert formulas and the Manning formula to grossly estimate flow rates. Of course, this method requires that depth of flow measurements be made in two manholes instead of one, thus doubling the cost of flow measurement.
Great advances have been made in the last decade in the design of equipment for measuring, recording, and storing depth of liquid flow (or pressure) information. No one, however, has developed a portable primary wastewater flow metering device that would allow depths of flow or pressure head to be accurately converted into flow rates under both free flow and submerged flow, open channel conditions and under full pipe, surcharged conditions.